Front truck on steam locomotives.

[tuba1st]

I have two 4-8-4s and a 4-6-4 on my layout.  The radius curves are 22" which the directions advise is the minimum.  The tracks are level and no matter what speed the front truck on each comes off the track at a certain portion of the curve.  If I run the engines in the opposite direction they stay on but counter clockwise they come off.  Any suggestions?

[GN.2-6-8-0]

Several things to look at" 1st check the wheel gauge of the leading truck wheels with a NMRA gauge,If they are in gauge take a very close look at your track work in the area where they are derailing and see if there are track joiners there make sure the rails are lined up perfectly also use the gauge to check and assure your track is in fact in gauge. You do not say if this problem area is accuring on a switch, If so these need to be looked at very closely,the lead wheels can and do easily pick the points of switch's if the trunout is not thrown completely.

[tuba1st]

I'll have to acquire a new gauge.  The old on disappeared when we moved.  At slow speed all of the engines handle the switches fine.  Its just at these two 22" curves.  I tried replacing the sections where the derail happens and the problem still exists.  The derail occurs after the connection between the tracks.

[Yampa Bob]

Hi Tuba
The typical pony truck mounting on these locomotives is problematic, it's sorta like trying to "push a rope".  In other words, the truck is being pushed by the mounting bar as compared to being pulled as in the case of a trailing truck.

Think about how easy it is to pull a small trailer behind a vehicle, but difficult to keep the trailer going straight if backing up. The trailer tries to "jack-knife".  Similarly when the loco starts into a curve, the front truck tries to jack-knife, it would prefer to be "pulled" rather than "pushed".

Since the truck derails only counterclockwise, it sounds like the right front wheel is not fully contacting the rail. A couple possibilities:
1.  The push bar or the truck might be slightly twisted.
2.  Either the rear pivot point or the pivot point at the truck center might be binding. The push bar and truck should swivel easily both directions.
3.  The inside edge of the railhead, of the outside rail, might have a rough spot at a joint.  Smoothness of the inside edges of railheads, especially at joints, is as important (if not more so) as smoothness of the top of rail. Anything a wheel flange can "pick" can cause a derail.  Any roughness you can feel with a fingernail needs to be smoothed.

Place the loco on a mirror, make sure all 4 wheels are contacting the glass equally. If you can slip a thin  piece of paper between a wheel and the glass the push bar or truck might be twisted.

Remove the screw holding the push bar to the chassis, check for any burrs or rough spots on the bar or chassis that might be causing a twist or bind.

The truck must be able to "rock" a bit, just as trucks on a freight car must rock to allow for irregularities in track and lateral forces.

Lastly, be sure both wheelsets roll easily, there could be a burr in the journal, or the journals press too tightly against the axle points.

If all else fails, try adding a tiny bit of weight on top of the truck. A small lead split shot (fishing tackle) can be flattened with a hammer, 1/16 ounce should suffice if there is room.

Hope this helps.

[tuba1st]

Thanks, I'll give a try after my daughter comes home and takes care of her daughter.

[ebtbob]

Good Morning All,

      Another thought......if you nail down you track,  make sure at the point of derailment that any track nail is not driven in too far.  This will cause the tie to curl upwards,  thus bringing the rails closer together,   track out of gauge.

Bob

[tuba1st]

Bob, I checked the track with a gauge and also the wheels sitting on a glass table.  Everything checked out.  It appears that the truck is really light and the spring that is between the chassis and the truck is also quite light.  It looks like some weight is needed to hold the truck down.  That is the next step.
Tom

[rogertra]

Bob, I checked the track with a gauge and also the wheels sitting on a glass table.  Everything checked out.  It appears that the truck is really light and the spring that is between the chassis and the truck is also quite light.  It looks like some weight is needed to hold the truck down.  That is the next step.
Tom

Remove the truck springs, front and rear.  I do this on all my steam locos.

Removing the truck springs also slightly improves traction.